The BLOCS is a unique prospective study of longitudinal course and outcome of 395 participants with OCD that will generate important new data of practical clinical significance for patients, families and treating clinicians. We seek five additional years of funding to test hypotheses of fundamental significance to our understanding of the psychopathology, etiology and treatment of OCD, hypotheses that can only be answered in using a longitudinal design. With this longitudinal dataset we will be able to test the validity of a novel conceptual model of the illness that posits two latent higher order constructs of OCD that underlie the motivational drive leading to OC symptoms, harm avoidance (HA) and incompleteness (INC). These constructs have demonstrated clinical face validity and are thought to be differentially associated with fear (HA) and reward (INC) circuitry. This innovative model, and the dimensional instruments that have been developed to test the model, will be important in future explorations of the complex genetics, biologically meaningful endophenotypes, and treatment outcome studies of the disorder. The BLOCS will provide information about prognosis, treatment received, and rates and predictors of remission and relapse that have significant implications for public health policy. We will investigate a number of previously unstudied questions related to maintenance treatment with SRIs including the characteristics of participants with sustained improvement, which will potentially elucidate what contributes to achieving and sustaining remission. We will also gather unique prospective data on the patterns of course and treatment received for child and adolescent and aging cohorts with OCD. Our specific aims are: (1) To evaluate the conceptual validity of a higher latent construct model of harm avoidance (HA) and incompleteness (INC), and to test the relative contributions of these latent constructs, primary symptom subtypes, OCD symptom factors, and temperament in explaining variability in course (2) collect comprehensive treatment data related to long term treatment and course (3) to determine the patterns of course and outcome of a clinical sample of children and adolescents with OCD. Subjects will be evaluated at 1 year intervals with instruments that obtain detailed information on symptom status and severity, diagnostic status, treatment received, psychosocial functioning, and other domains. We propose to add new assessments that measure key latent variables, underlying mood- and anxiety-related traits, and symptom severity. We have also incorporated a structural equation model into our longitudinal data analysis to test novel hypotheses related to the explanatory power of symptoms, temperament and the higher order latent constructs (HA and INC) on the variance of course. To have sufficient statistical power to test our hypotheses, 5 more years of prospective observation and 50 additional child and adolescent participants are needed. Continuation of BLOCS will shed new light on clinically and theoretically important, innovative questions about an understudied major psychiatric disorder which have not been addressed by previous research.